For many years, metal roofing systems, specifically stainless steel and low carbon steel sheet in various sheet gauge thicknesses, have been treated with terne metal alloys. When the terne coated steel sheets are assembled a roof covering, adjacent sheet edges are folded over one another and the seam then formed, typically a standing seam, usually soldered vis-a-vis the terne coating to produce a waterproof joint. Today, the terne coated steel sheets are either preformed or formed at the job site onto roofing pans with bent edges which abut edges of adjacent pans which are then pressed or rolled into the seam. Similarly, caps, cleats, etc. are likewise formed from the terne coated sheet. In addition to providing for soldering of the seams, the terne coating inhibits rusting or oxidation of the metal sheet which would otherwise occur over time.
Terne or terne alloy is a term commonly used to describe an alloy containing about 80% lead and the remainder tin. The terne alloy is conventionally applied to the metals by a hot dip process wherein the metal is immersed into a molten bath of terne metal. The terne coating greatly inhibits the formation of ferrous oxide on the metal thus preventing corrosion and extending the life of the metal. The corrosion resistive properties of the terne alloy are due to the stability of elemental lead and tin and the lead-tin oxide which forms from atmospheric exposure.
Although terne coated sheet metals have excellent corrosive resistive properties and have been used in various applications such as roofing, terne coated metal roofing materials have recently been questioned due to environmental concerns. Terne coated metals contain a very high percentage of lead and commonly include over 80 weight percent of the terne alloy. Although the lead in terne alloys is stabilized, there is concern about leaching of the lead from the terne alloy. As a result, terne coated materials have been limited from use in various applications, such as aquifer roofing systems. The concern of lead possibly leaching from terne coated roofing systems renders normal terne coating inadequate and undesirable as a metal roofing coating for these types of roofing applications.
Another disadvantage of terne coated materials is the softness of the terne layer. As noted, terne coated metal sheets are commonly formed into varying shapes. The machines that bend the metal sheets periodically damage the terne coating during bending process. The terne coating is susceptible to damage due to the abrasive nature of the forming machines.
A further disadvantage of using normal terne coated metals is that newly applied terne is highly reflective to light. Use of terne roofing materials on buildings near or within an airport can produce a certain amount of glare to pilots taking-off and landing. Due to the highly stable nature of terne alloys, terne coated metals take about one and one-half to two years before oxidation of the terne begins to dull the terne alloy surface. The present invention deals with these disadvantage of normal terne coated roofing sheet material.